This invention relates to new and improved methods of perforating a cemented well bore casing and the surrounding cement.
In the process of establishing an oil or gas well, the well is typically provided with an arrangement for selectively establishing fluid communication with certain zones in the formation traversed by the well. A typical method of controlling the zones with which the well is in fluid communication is by running well casing into the well and then sealing the annulus between the exterior of the casing and the walls of the wellbore with cement. Often the casing is expanded once it is run-in to the well. Thereafter, the well casing and cement may be perforated using mechanical or chemical means at preselected locations by a perforating device or the like to establish a plurality of fluid flow paths between the pipe and the product bearing zones in the formation.
Much effort has been devoted to developing apparatus and methods of perforation. Explosive charges are sometimes used to construct perforating guns, such as disclosed in U.S. Pat. No. 5,701,964 to Walker et al. Attempts have been made to increase the effectiveness of explosive perforation methods by combining them with propellant fracture devices. An example of such attempts is disclosed in U.S. Pat. No. 5,775,426 to Snider, et al, wherein a sheath of propellant material is positioned to substantially encircle at least one shaped charge. Under this method, the propellant generates high-pressure gasses, which clean the perforations left by explosive charges.
Problems exist with the use of explosives to perforate casing, however. Unfortunately, the process of perforating through the casing and then though the layer of cement dissipates a substantial portion of the energy from the explosive perforating device and the formation receives only a minor portion of the perforating energy.
Further, explosives create high-energy plasma that can penetrate the wall of the adjacent casing, cement sheath outside the casing, and the surrounding formation rock to provide a flow path for formation fluids. Unfortunately, the act of creating the perforation tunnel may also create some significant debris and due to the force of the expanding plasma jet, drive some of the debris into the surrounding rock thereby plugging the newly created flow tunnel. Techniques have been developed to reduce the effect of the embedded debris, such as performing the perforation operation in an under-balanced condition or performing backflushing operations following perforation.
Perforating in an under-balanced condition causes the formation fluids to surge into the wellbore yielding a cleaning effect. After perforating in an under-balanced condition the well must be xe2x80x9ckilledxe2x80x9d by circulating out the produced fluids and replacing them with heavier completion fluids. Oftentimes significant amounts of completion fluid are then lost to the formation, which can be expensive and potentially damaging to productivity. Fluid loss may result in formation damage due to swelling of formation clay minerals, particle invasion into the formation, dissolution of matrix cementation thereby promoting fines migration, and by interaction between the completion fluids and the formation fluids causing emulsion or water blocks or changes in the wetability of the formation sand. Fluid loss pills may also be required, which can be expensive and damaging.
Mechanical perforation may avoid many of these problems. Devices for mechanically perforating a well casing without the use of explosives are also known in the art and, in fact, predate the use of explosives. Laterally movable punches are exemplified by the devices shown in the Jobe, U.S. Pat. No. 2,482,913, Frogge, U.S. Pat. No. 3,212,580, Grable, U.S. Pat. No. 3,720,262, and Gardner, U.S. Pat. No. 4,165,784, which are each incorporated herein by reference. Toothed wheel perforators are exemplified by the devices showing in Graham, U.S. Pat. No. 1,162,601; Noble, U.S. Pat. No. 1,247,140; Baash, U.S. Pat. No. 1,259,340; Baash, U.S. Pat. No. 1,272,597; Layne, U.S. Pat. No. 1,497,919; Layne, U.S. Pat. No. 1,500,829; Layne, U.S. Pat. No. 1,532,592; Jerome, U.S. Pat. No. 4,106,561; and Hank, U.S. Pat. No. 4,220,201, which are each incorporated herein by reference.
It is also known in the art to run into a well a liner that is pre-perforated with the openings filled by shearable plugs. Such a device is exemplified by U.S. Pat. No. 4,498,543 to Pye, which is incorporated herein by reference.
Unfortunately, these mechanical and shearable plug methods of perforation are of limited use where the casing is cemented in place and these methods do not perforate the fluid bearing formation.
Method and apparatus are presented for perforating a subterranean formation so as to establish fluid communication between the formation and a wellbore, the wellbore having casing cemented therein, the casing having a cement sheath therearound. The casing is perforated with a mechanical perforator and thereafter a propellant material is ignited within the casing thereby perforating the cement sheath. The formation may thereafter be stimulated with an acid stimulator. The mechanical perforator may include use of a toothed wheel, or a needle-punch perforator. The propellant may be deployed in a sleeve and may comprise an abrasive material.